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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.astro.expert
Subject: Re: What Constitutes A "DIMENSION"? {Question From a Novice}
Message-ID: <201@utastro.UUCP>
Date: Thu, 6-Jun-85 18:18:51 EDT
Article-I.D.: utastro.201
Posted: Thu Jun  6 18:18:51 1985
Date-Received: Sun, 9-Jun-85 01:45:29 EDT
References: <293@ihlpa.UUCP>
Distribution: net
Organization: U. Texas, Astronomy, Austin, TX
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[]
Steve Aldrich writes

> With all the recent discussion on the "Center of the {Known} Universe,"
> and/or 3D(+) Space-Time, I thought I'd pose a question of my own.
> 
>  Namely WHAT, to the best of current knowledge, exactly is a dimension?
> How are they defined, and what seperates them from the others? Is there
> a "perceptable" border across dimensions, and if so what constitutes the
> "barrier region" between them? Is it a "state of energy/matter" or am I
> completely off base?

A dimension is a continuous parameter necessary to uniquely define an
event in space-time.  "Space has three dimensions" means that on
any spatial hypersurface three numbers are needed.  A particular direction
associated with a dimension is a matter of convention (in the above example).
"Space-time has four dimensions" just means to add another number (the date).
The direction of the time direction is not *completely* arbitrary, but
is substantially so.  Observers moving at different speeds will define
different time "directions" in space-time.

Science fiction writers have used the word in a slightly different sense.
The idea is that space-time might have umpteen dimensions, only four of
which are readily apparent.  Then something that is near us *in an
unusual direction* is referred to as another "dimension",  (as in
"The Giant Cockroach from the Twelfth Dimension that Ate Detroit").
-- 

"Don't argue with a fool.      Ethan Vishniac
 Borrow his money."            {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               Department of Astronomy
                               University of Texas